The chemokine CXCL12 (also called SDF-1 - stromal-derived-factor-1) and its receptor, CXCR4 ((CXC Chemokine Receptor 4), a molecule endowed with potent chemotactic activity for hematopoietic cells, together play a pivotal role in the trafficking of hematopoietic stem cells to the bone marrow 10,11. The CXCL12/CXCR4 axis is also critically involved in the retention of hematopoietic cells within the BM microenvironment. Following chemotherapy and irradiation, the CXCL12/CXCR4 axis delays the recovery of the BM progenitor cells and the exit of mature cells such as neutrophils and monocytes to the periphery. Consequently, disruption of the CXCL12/CXCR4 interaction results in mobilization of hematopoietic cells including stem cells and a faster recovery of the treated BM.